In most TCVRs the colling fan is
triggered together with PTT. This makes sence as the most of the
heat is produced during the transmission. Also the noise produced
by the fan is less disturbing during TX.
The IC-275 doesn't work like this, however. The fan doesn't run
during TX normally. The heat is absorbed by the massive aluminium
heatsink hidden under the top cover. When the heatskink reaches
50ºC the fan starts colling. Then the fan runs all the time, at
lower speed during RX, at higher speed during TX, until the
temperature falls below 50ºC again.
This way you can have a feeling that the TCVR is extremely quiet
when you use it for occasional short QSOs over the local
repeater, or something like that. If you use it for 24-hours
lasting contest this behaviour is not much convenient.
I use IC-275 in very adverse conditions. Besides that I need to
CQ for long tens of minutes or even hours during contests, I use
it often in an ambient temperature above 30º C. I needed to make
the cooling as efficient as possible.
The following modification changes the behaviour in the following
manner. During RX the fan doesn't run. During TX the fan runs at
high speed. After the TX -> RX transition the fan runs at a
lower speed for about 7 seconds, then it switches off. To protect
the transceiver if the temperature of the heatsink reaches 50º C
the fan runs at high speed regardless RX/TX status.